To determine the mechanism by which T cells are educated in the thymus, neonatal mice were chronically treated with monoclonal anti-IA kappa antibodies in vivo. The results of these studies demonstrate that such mice are virtually devoid of Ia+cells. The T cells from these mice were found to be deficient in their recognition of either syngeneic or allogeneic class II MHC determinants, but were not deficient in their recognition of either syngeneic or allogeneic class I MHC determinants. The defect in Ia recognition correlated precisely with the intra-thymic suppression of Ia antigen expression but did not correlate with the extra-thymic suppression of Ia antigen expression. It is concluded that Ia-specific and K/D-specific T cells are educated on different intra-thymic elements.